Seizure-like thalamocortical rhythms initiate in the deep layers of the cortex in a co-culture model

Astrocytic Regulation of Synchronous Bursting in Cortical Cultures: From Local to Global

Synchronous bursting (SB) is ubiquitous in neuronal networks and independent of network structure. Although it is known to be driven by glutamatergic neurotransmissions, its underlying mechanism remains unclear. Recent studies show that local glutamate recycle by astrocytes affects nearby neuronal activities, which indicate that the local dynamics might also be the origin of SBs in networks. We investigated the effects of local glutamate dynamics on SBs in both cultures developed on multielectrode array (MEA) systems and a tripartite synapse simulation. Local glutamate uptake by astrocytes was altered by pharmacological targeting of GLT-1 glutamate transporters, whereas neuronal firing activities and synaptic glutamate level was simultaneously monitored with MEA and astrocyte-specific glutamate sensors (intensity-based glutamate-sensing fluorescent reporter), respectively. Global SB properties were significantly altered on targeting GLT-1. Detailed simulation of a network with astrocytic glutamate uptake and recycle mechanisms, conforming with the experimental observations, shows that astrocytes function as a slow negative feedback to neuronal activities in the network. SB in the network can be realized as an alternation between positive and negative feedback in the neurons and astrocytes, respectively. An understanding of glutamate trafficking dynamics is of general application to explain how astrocyte malfunction can result in pathological seizure-like phenomena in neuronal systems.

Automated quantification of neuronal networks and single-cell calcium dynamics using calcium imaging

BACKGROUND

Recent advances in genetically engineered calcium and membrane potential indicators provide the potential to estimate the activation dynamics of individual neurons within larger, mesoscale networks (100s-1000+neurons). However, a fully integrated automated workflow for the analysis and visualization of neural microcircuits from high speed fluorescence imaging data is lacking.

NEW METHOD

Here we introduce FluoroSNNAP, Fluorescence Single Neuron and Network Analysis Package. FluoroSNNAP is an open-source, interactive software developed in MATLAB for automated quantification of numerous biologically relevant features of both the calcium dynamics of single-cells and network activity patterns. FluoroSNNAP integrates and improves upon existing tools for spike detection, synchronization analysis, and inference of functional connectivity, making it most useful to experimentalists with little or no programming knowledge.

RESULTS

We apply FluoroSNNAP to characterize the activity patterns of neuronal microcircuits undergoing developmental maturation in vitro. Separately, we highlight the utility of single-cell analysis for phenotyping a mixed population of neurons expressing a human mutant variant of the microtubule associated protein tau and wild-type tau.

COMPARISON WITH EXISTING METHOD(S)

We show the performance of semi-automated cell segmentation using spatiotemporal independent component analysis and significant improvement in detecting calcium transients using a template-based algorithm in comparison to peak-based or wavelet-based detection methods. Our software further enables automated analysis of microcircuits, which is an improvement over existing methods.

CONCLUSIONS

We expect the dissemination of this software will facilitate a comprehensive analysis of neuronal networks, promoting the rapid interrogation of circuits in health and disease.

Selective pharmacological manipulation of cortical-thalamic co-cultures in a dual-compartment device

In this study, we demonstrate capabilities to selectively manipulate dissociated co-cultures of neurons plated in dual-compartment devices. Synaptic receptor antagonists and tetrodotoxin solutions were used to selectively control and study the network-wide burst propagation and cell firing in cortical-cortical and cortical-thalamic co-culture systems. The results show that in cortical-thalamic dissociated co-cultures, burst events initiate in the cortical region and propagate to the thalamic region and the burst events in thalamic region can be controlled by blocking the synaptic receptors in the cortical region. Whereas, in cortical-cortical co-culture system, one of the region acts as a site of burst initiation and facilitate propagation of bursts in the entire network. Tetrodotoxin, a sodium channel blocker, when applied to either of the regions blocks the firing of neurons in that particular region with significant influence on the firing of neurons in the other region. The results demonstrate selective pharmacological manipulation capabilities of co-cultures in a dual compartment device and helps understand the effects of neuroactive compounds on networks derived from specific CNS tissues and the dynamic interaction between them.

Enhanced NMDA receptor-dependent thalamic excitation and network oscillations in stargazer mice

Disturbances in corticothalamic circuitry can lead to absence epilepsy. The reticular thalamic nucleus (RTN) plays a pivotal role in that it receives excitation from cortex and thalamus and, when strongly activated, can generate excessive inhibitory output and epileptic thalamocortical oscillations that depend on postinhibitory rebound. Stargazer (stg) mice have prominent absence seizures resulting from a mutant form of the AMPAR auxiliary protein stargazin. Reduced AMPAR excitation in RTN has been demonstrated previously in stg, yet the mechanisms leading from RTN hypoexcitation to epilepsy are unknown and unexpected because thalamic epileptiform oscillatory activity requires AMPARs. We demonstrate hyperexcitability in stg thalamic slices and further characterize the various excitatory inputs to RTN using electrical stimulation and laser scanning photostimulation. Patch-clamp recordings of spontaneous and evoked EPSCs in RTN neurons demonstrate reduced amplitude and increased duration of the AMPAR component with an increased amplitude NMDAR component. Short 200 Hz stimulus trains evoked a gradual approximately threefold increase in NMDAR EPSCs compared with single stimuli in wild-type (WT), indicating progressive NMDAR recruitment, whereas in stg cells, NMDAR responses were nearly maximal with single stimuli. Array tomography revealed lower synaptic, but higher perisynaptic, AMPAR density in stg RTN. Increasing NMDAR activity via reduced [Mg2+]o in WT phenocopied the thalamic hyperexcitability observed in stg, whereas changing [Mg2+]o had no effect on stg slices. These findings suggest that, in stg, a trafficking defect in synaptic AMPARs in RTN cells leads to a compensatory increase in synaptic NMDARs and enhanced thalamic excitability.

Focal administration of neuropeptide Y into the S2 somatosensory cortex maximally suppresses absence seizures in a genetic rat model

PURPOSE

Neuropeptide Y (NPY) is an inhibitory neurotransmitter that suppresses focal and generalized seizures in animal models. In this study, we investigated the sites within the thalamocortical circuit that NPY acts to suppress seizures in genetic absence epilepsy rats from Strasbourg (GAERS).

METHODS

In conscious freely moving GAERS, NPY was administered via intracerebral microcannulae implanted bilaterally into one of the following regions: primary somatosensory cortex (S1), secondary somatosensory cortex (S2), the primary motor cortex (M1), caudal nucleus reticular thalamus (nRT), or ventrobasal thalamus (VB). Animals received vehicle and up to three doses of NPY, in a randomized order. Electroencephalography (EEG) recordings were carried out for 30 min prior to injection and 90 min after the injection of NPY or vehicle.

KEY FINDINGS

Focal microinjections of NPY into the S2 cortex suppressed seizures in a dose-dependent manner, with the response being significantly different at the highest dose (1.5 mm) compared to vehicle for total time in seizures postinjection (7.2 ± 3.0% of saline, p < 0.01) and average number of seizures (9.4 ± 4.9% of saline, p < 0.05). In contrast NPY microinjections into the VB resulted in an aggravation of seizures.

SIGNIFICANCE

NPY produces contrasting effects on absence-like seizures in GAERS depending on the site of injection within the thalamocortical circuit. The S2 is the site at which NPY most potently acts to suppress absence-like seizures in GAERS, whereas seizure-aggravating effects are seen in the VB. These results provide further evidence to support the proposition that these electroclinically "generalized" seizures are being driven by a topographically restricted region within the somatosensory cortex.